The present invention pertains generally to systems and methods for analyzing diffraction patterns obtained from an electron microscope and, more particularly, to a method and apparatus for correcting distortions in diffraction patterns obtained from crystalline specimens using an electron microscope.
Scanning electron microscopes (SEMs) are used to investigate and characterize features within sample materials of interest. For example, an SEM may be used to obtain crystallographic information, e.g., the size and shape of constituent crystals or grains, the orientation of crystal lattices, and the spatial location of the crystals within a polycrystalline material. Based on such crystallographic information, the properties and characteristics of the material may be determined. Such information is useful for understanding why certain crystalline materials behave as they do, to predict how other materials will behave, and to alter or to otherwise control material forming and processing techniques to improve specific material properties.
A typical procedure for determining the crystallographic characteristics of a polycrystalline sample specimen involves bombarding selected points of the specimen with a beam of electrons produced by an SEM. The electrons interact with a small volume of the material sample at the selected points, and diffracting crystals cause electron backscatter diffraction (EBSD) patterns to form on a detector, e.g., a phosphorous screen, placed near the specimen in the SEM. The EBSD patterns may be imaged through a video camera, and digitized for further processing. Good quality EBSD patterns include a number of intersecting, relatively high intensity bands that are usually referred to as Kikuchi bands, which result from electrons being diffracted from various planes in the crystal lattice at the point of bombardment. An abundance of microstructure information may be obtained by analyzing the various parameters of the Kikuchi bands. Computer-implemented image processing techniques have been developed to analyze Kikuchi bands from the EBSD patterns taken at numerous points on a material sample, and to generate displays of the crystalline specimen that convey a wealth of microstructure information.
A problem arises when EBSD patterns are obtained using certain SEMs (immersion-lens SEMs) which employ a final (objective) electron lens, for focusing and directing the electron beam, which produces large magnetic fields near the sample being analyzed. Although these magnetic fields are required for superior image resolution, they are detrimental to quality EBSD pattern formation in that the fields distort the near-linear trajectory of the electrons emerging from the sample, thereby distorting the EBSD pattern which is detected by the detector. EBSD patterns are distorted by these magnetic fields such that features that should appear straight, if detected using a conventional SEM which does not produce such fields, are curved in the EBSD pattern images obtained using an SEM which does produce such fields. Accurate analysis of such distorted EBSD pattern images is impossible. Thus, SEMs which produce magnetic fields which distort EBSD patterns cannot be used for crystallographic structure analysis unless the distortion in the EBSD pattern can be avoided or corrected.
In general, users tend to avoid using SEMs which produce magnetic fields which distort EBSD patterns for EBSD pattern analysis. This is unfortunate, since such SEMs otherwise provide superior image resolution. If EBSD pattern analysis systems are employed with such SEMs, abnormal geometric mounting of the SEM electron beam generator and EBSD pattern collection system may be possible to avoid magnetic field distortion of the EBSD pattern image. However, such a modification will be at the expense of SEM imaging quality and EBSD pattern collection performance. Since the magnetic field strengths and distributions employed in SEMs are highly guarded secrets of the various SEM manufacturers, and are not generally available to the public or to developers of EBSD pattern analysis systems, correction of distorted EBSD pattern images cannot come from physical modeling of the distorting magnetic fields.
What is desired, therefore, is a system and method for correcting automatically the distortion in EBSD pattern images obtained using SEMs which generate pattern distorting magnetic fields, which is based only upon available empirical information, and which produces corrected EBSD pattern images which may be analyzed using conventional EBSD pattern analysis techniques.
The present invention provides a system and method for correcting automatically the distortions in EBSD patterns which result from the magnetic fields generated in some SEMs used for collecting such patterns. A method for correcting magnetic field distortions in an EBSD pattern in accordance with the present invention may be implemented as a software program running on a computer which is part of a conventional system for obtaining and analyzing EBSD patterns. The first time that the correction method is run for a particular SEM geometry, a calibration procedure is run to obtain pattern distortion correction parameters based on a single or multiple mathematical curves extracted from a calibration image. These correction parameters may be employed each time an EBSD pattern image is obtained using this SEM geometry to correct automatically any magnetic field distortion in the EBSD pattern. Traditional EBSD pattern analysis methods may be used to analyze such corrected EBSD patterns to obtain crystallographic structure information for a sample being analyzed. The present invention provides for correcting automatically a distorted EBSD pattern based solely on empirical information. Thus, the present invention may be employed in combination with any SEM used for EBSD pattern collection without the need for any data on the particular magnetic field strengths and distributions employed in the SEM. Furthermore, the present invention may be used to obtain distortion-free EBSD patterns for analysis without abnormal geometric mounting of the SEM electron beam generator, sample, and pattern collection system, which might adversely affect SEM imaging and EBSD pattern collection performance.
In accordance with the present invention, an EBSD pattern may be obtained in a conventional manner using a conventional EBSD pattern collection and analysis system. For example, a sample, e.g., of a polycrystalline material, may be mounted in an SEM and bombarded by an electron beam. Backscattered electrons from the sample are detected, in a conventional manner, by a detection and imaging system including a detector, e.g., a phosphorous screen, a camera, for recording the image produced by the detector, and a digitizer, for digitizing the video image and providing it to a computer system for EBSD pattern analysis. Any distortion in the EBSD pattern image which results from the magnetic fields produced by the final (objective) electron lens in the SEM must be corrected before analysis of the EBSD pattern is performed. In accordance with the present invention, the distorted EBSD pattern may be corrected by a correction method implemented as a software program, which may be run on the same computer which is employed for EBSD pattern analysis.
In accordance with the present invention, a distorted EBSD pattern is corrected using correction parameters based on a single or multiple mathematical curves which are used to shift the intensity values in the distorted EBSD pattern image, pixel line by pixel line, to remove any magnetic field distortion therefrom to provide as nearly a distortion-free image as possible. The correction patterns are obtained by performing a calibration procedure. Once the calibration procedure has been performed, the correction parameters obtained may be used to correct the distortion in all subsequent EBSD pattern images obtained using the particular SEM geometry for which the calibration procedure was run.
In the calibration procedure, a distorted EBSD pattern is obtained in a conventional manner as described above from a known crystalline material calibration sample. For example, a silicon calibration sample  less than 100 greater than  with the low index direction mounted vertically in the SEM may be used. The EBSD pattern thus obtained, which is distorted by magnetic fields in the SEM, is displayed on an operator display. The operator display, in combination with a user input device, such as a mouse, forms a user interface for the calibration procedure. Using the user input device, e.g., the mouse, an operator defines segment endpoints along a low index Kikuchi band (represented by the low index direction) in the distorted EBSD pattern image displayed. The result is a segmented curved line following the curved Kikuchi band in the distorted EBSD pattern. In a non-distorted EBSD pattern, the Kikuchi band would follow a straight line. From the user-defined segment endpoints, the calibration procedure calculates a single or a series of mathematical curves to fit. For example, a cubic spline or polynomial curve may be calculated. The mathematical curve or curves define the amounts by which points along the user-defined curved line must be shifted in order to form a straight line. These correction parameters are saved into a pattern correction parameter data file. Alternatively, the correction parameters may be calculated in a more automated fashion.
The correction parameters obtained in the calibration procedure are employed in a correction procedure to correct the magnetic field distortions in EBSD patterns collected using the SEM geometry for which the calibration procedure was performed. Having obtained an EBSD pattern in a conventional manner using the distorting SEM system, the stored correction parameters are retrieved and are employed to shift lines of pixels in the distorted EBSD image by an amount defined by the correction parameters to correct the distortion in the EBSD pattern image. For example, the intensities of each line in the distorted EBSD pattern image may be shifted, row-by-row, by the amount determined by the mathematical curve calculation for each vertical position in the image. Any xe2x80x9cunfilledxe2x80x9d region of the EBSD pattern image which is created by the shifting operation may be filled with an intensity equal to the average intensity of the whole image (thereby creating a curved xe2x80x9cwedgexe2x80x9d feature along one side of the image). The thus corrected EBSD pattern image may be displayed to an operator of the system, and saved for subsequent EBSD pattern analysis using conventional EBSD pattern analysis techniques.
Further objects, features, and advantages of the present invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings.